Pantothenate kinase-associated neurodegeneration (PKAN) is a rare genetic neurodegenerative disease. The disease is caused by mutations in the gene coding for pantothenate kinase 2 (PANK2). Pantothenate kinase 2 phosphorylates pantothenate to 4′-phosphopantothenate in the de novo biosynthesis pathway of coenzyme A. In PKAN patients, PANK2 (one of the four PANK isoforms known in humans and localized to the mitochondria) is affected and leads to severe neurodegeneration and premature death (Zhou, B. et al. Nat. Genet., 2001, 28, 345).
Pantothenate, 4′-phosphopantothenate, pantetheine and 4′-phosphopantetheine have all been suggested as potential agents for the treatment of PKAN. For example, 4′-phosphopantothenate and 4′-phosphopantetheine, both intermediates of the CoA metabolic pathway downstream of the pantothenate kinase step, were envisioned as potential treatment options for PKAN (Zhou, B. et al. Nat. Genet., 2001, 28, 345, WO 2003/008626). Pantethine (the disulfide of pantetheine) has been shown to rescue a Drosophila model of PKAN (Rana, A. et al., PNAS, 2010, 107, 6988).
The use and testing of phosphorylated derivatives of pantothenate and pantetheine is hampered by a lack of suitable methods of preparation and purification procedures. Most research on the synthesis of various phosphorylated pantothenic acid derivatives has been done a long time ago, when the availability of analytical techniques was limited. Therefore, in many cases the structure and purity of products were just assumed, but not otherwise established. Moreover, in most cases the procedures are not well described and their reproducibility is questionable (King, T. E. et al. J. Biol. Chem., 1951, 191, 515; King, T. E. Science, 1950, 112, 562; J. Baddiley and E. M. Thain, J. Chem. Soc., 1953, 1610; Kopelevich, V. M., Khim. Farm. Zh., 1967, 11, 26; Hashimoto, Chem. Lett., 1972, 595).
Recently, a chemical synthesis of 4′-phosphopantothenate was described. The described synthesis method is a tedious 6-step synthesis procedure (Strauss et al., Biochemistry, 2004, 43, 15520). In the same publication an older synthesis method for 4′-phosphopantetheine and 4′-phosphopantothenoyl cysteine is disclosed.
Although pantethine is a potent compound rescuing the Drosophila PKAN model, in serum pantethine is rapidly converted by pantetheinases to vitamin B5 and cysteamine (Wittwer et al., J. Clin. Invest., 1985, 76, 1665) and therefore the compound pantethine is less likely to be an effective treatment for PKAN. This is confirmed by our own unpublished observations.
While a medical use of 4′-phosophopantetheine has already been speculated upon, a medical use of (S)-acyl-4′-phosphopantetheine derivatives has heretofore not been envisioned. In particular, it has not been suggested that such compounds could be useful in the treatment of neurodegenerative diseases, such as PKAN.
As a member of the group of (S)-acyl-4′-phosphopantetheine derivatives, (S)-acetyl-4′-phosphopantetheine has been structurally described (Lee, C-H. and Sarma, R. H., JACS, 1975, 97, 1225). However, an economically viable chemical synthesis method, or a possible medical use have not been reported for this compound.
(S)-benzoyl-4′-phosphopantetheine has been described as an intermediate in the synthesis of CoA derivatives (WO2012/17400). However, a pharmaceutical use of this compound was not envisaged.
Methods to increase the efficacy of compounds in medical treatment are known, in particular methods to increase the ability of pharmaceutical compounds to penetrate through membranes, or the blood-brain barrier. The medical use of phosphorylated compounds was often found to be limited by the compounds' poor ability to penetrate cell membranes. This has been attributed to the negative charge on the phosphate group which does not easily pass through cell membranes. Therefore, masking of the phosphate group, to yield a neutralized form, and to use the masked compound as a prodrug, allows the delivery of the medicament to the interior of the cells, where esterases present in the cells may subsequently cleave the protection groups from the phosphate groups, to release the active form of the drug. Thereby the bioavailability of biologically active phosphorylated compounds can be increased (Schultz, K. Bioorg. Med. Chem., 2003, 11, 885). Commonly used masking groups for this purpose are acyloxyalkyl groups, such as pivaloyloxymethyl (POM) and acetoxymethyl (AM). POM derivatives specifically have been shown to be stable in buffer and plasma.